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Estuaries are under intense pressure, because of urban developments and water abstraction. Water column characteristics of the Nahoon Estuary were examined in 2014–2015 to identify factors that influenced phytoplankton dynamics. The estuary was found to be saline (∼33) and remained well-oxygenated (∼6 mg l−1) throughout the current study. During this period, freshwater inflow into the estuary was minimal and chlorophyllabiomass was low (3.6 ± 0.3 µg l−1). Nutrient concentration at Nahoon Estuary showed a linear relationship with salinity, indicative of a conservative behaviour. Phytoplankton dynamics were altered by fluctuations in nutrient levels, not necessarily in response to freshwater inflow. The impact of Nahoon Dam and destructive land-use alongside the Nahoon River decreases freshwater inflow and increases nutrient loading into the estuary. In terms of nutrient loading, the current study showed the importance of microalgae as indicators of the nutrient status of an estuary. The presence of cyanobacteria and high phytoplankton biomass in the upper reaches of the estuary confirms the input of excessive nutrients. The results of the current study clearly show that better management of both the Nahoon River and its catchment is vital to ensure the health of the system. [ABSTRACT FROM PUBLISHER]

The role of microbial diversity in ecosystem functioning is becoming increasingly recognized. Microalgae are a large group of organisms that are extremely diverse and heterogeneous from evolutionary and ecological viewpoints. However, estimates of their diversity are nebulous and require substantial input from systematics, dispersal analyses, and biogeography. Our generally poor understanding of microalgal biogeography has generated several hypotheses. According to one view, microalgae are cosmopolitan. However, use of advanced morphometric and molecular techniques suggest a restricted distribution (i.e., endemism) for many microalgae that were previously considered as cosmopolitan. The quest to have an exclusive theory of microalgal biogeography depends upon efforts to solve the problems related with appropriate taxonomic unit, genotype-morphospecies relationship, extent of synonymy in known species, degree of coexistence of endemic and cosmopolitan species, reasons for apparent restricted distribution, and the relative importance of sympatric speciation in genotype variations. This review discusses microalgal diversity from a biogeographical perspective, which, in turn, is analyzed in the light of empirical evidence available for the group. Also discussed are the arguments emerging from ongoing debates on the biogeography of other microorganisms. In addition to their immense ecological importance, microalgae have emerged as a potential source for a range of precious products including neutraceuticals, energy, and therapeutic drugs. Therefore, an understanding of their diversity and spatial distribution is crucial for formulating the conservation strategies for the group. [ABSTRACT FROM AUTHOR]